1. Field of the Invention
The present invention relates to addition interpolymers containing blocked isocyanates. More specifically, it relates to curable coating compositions comprising acrylic polymers containing blocked isocyanates.
1. Brief Description of the Prior Art
Coating compositions comprising acrylic polymers and blocked isocyanate curing agents are known in the art. The polymers, generally, contain active hydrogens, such as hydroxyl groups. The blocked isocyanates are stable at ambient temperatures but unblock at higher temperatures to bring about a crosslinking or curing reaction.
Examples of prior art are U.S. Pat. Nos. 4,177,178 and 3,939,051. For example, U.S. Pat. No. 4,177,178 discloses aqueous thermosetting resinous compositions comprising acrylic polymers containing active hydrogens and fully blocked diisocyanate curing agents. U.S. Pat. No. 3,939,051 discloses cationic electrodepositable coating compositions comprising a copolymer derived from monomers, at least one of which is a reaction product of a diisocyanate of which one of the isocyanato groups is "half-blocked" by reacting it with an ethylenically unsaturated hydroxyl-containing monomer followed by "fully-blocking" by reacting the other isocyanato group with a saturated alcohol. Although cured films obtained from these coating compositions exhibit desirable properties of hardness and solvent resistance, the coating compositions, for one reason or the other, do have some of the following disadvantages.
In coating compositions such as described in U.S. Pat. No. 4,177,178, the fully blocked polyisocyanates do not constitute an integral part of the film former. There may, therefore, be a differential in rate of migration of the film former and the blocked diisocyanates toward a workpiece that is being coated. A disproportionate migration of the blocked diisocyanates would adversely affect cure. In view of the foregoing, polymers with integral isocyanates have been formulated. The formulation of polymers with integral isocyanates, such as disclosed in U.S. Pat. No. 3,939,051, however, presents problems affecting the preparation and stability of the intended polymer.
Of concern, here, are problems attendant upon the use of blocked polyisocyanato monomers in preparing addition interpolymers, particularly acrylic polymers containing integral isocyanato groups. It is believed that wherein the process for preparing these polymers involves a step of "half-blocking" a diisocyanate with a blocking agent such as an alcohol, the "half-blocking" reaction yields a mixture of "half-blocked", unblocked and fully blocked products. A portion of the unblocked diisocyanato products react with the hydroxyl-containing monomer to form diethylenically unsaturated moieties which may result in gelation during the polymer preparation. Further, if there is some unblocked product present and one of the monomers used in preparing the polymer is an active hydrogen group-containing monomer, premature gelation may also result.
The present invention overcomes this concern by providing nongelled polymers with integral isocyanato groups which avoid the aforementioned problems and which can be formulated into coating compositions.